Polycarbonate is a useful engineering thermoplastic because it possesses, in general, a combination of several different favorable properties. Certain polycarbonates which have a glass transition temperature exceeding at least 165.degree. C., known as "high heat" polycarbonates, additionally have better resistance to thermal deformation than others. However, it is often found that a high heat polycarbonate is notch sensitive to impact loading, and often is undesirably subject to failure by brittle fracture. Brittle fracture, in this context, is usually indicated when a point of applied stress where fracture occurs exhibits smooth fracture surfaces, does not exhibit shear lips, and is not hinged. As a consequence of its brittleness, a high heat polycarbonate typically exhibits a low level of toughness, manifested particularly as notch sensitivity to impact loading, at a sufficiently high level to outweigh the benefits which would otherwise be obtainable from its superior resistance to thermal deformation.
Previous efforts to modify polycarbonate by blending it with materials such as a polyester, or polyester and an elastomeric impact modifier, as disclosed for example by U.S. Pat. Nos. 3,864,428 and 4,654,400, fail to address the special problems posed when a material possessing the brittleness, heat resistance and high viscosity of a high heat polycarbonate is incorporated into a blend. When a blend is formed form polyester and a polycarbonate which has a T.sub.g of about 150.degree. C. or less, the brittleness which would be present in the resulting blend because of the presence of the polyester can be more readily offset by the addition of an elastomer because the matrix provided by a polycarbonate which does not possess exceptionally high thermal resistance is ductile in and of itself. Experience with such varities of ductile polycarbonate does not therefore furnish any guidance concerning reducing the brittleness of a high heat polycarbonate.
Polyester tends to be a brittle, notch-sensitive material itself, and even an elastomeric impact modifier is not always adequate to significantly reduce the inherent brittleness of a blend of polyester with a high heat polycarbonate. Moreover, regardless of the system selected to reduce the notch sensitivity to impact loading of a high heat polycarbonate composition, the high viscosity inherent in a high heat polycarbonate requires that, in preparation of a composition, a high heat polycarbonate be selected which will allow for easy and convenient processing on known types of equipment.
It would accordingly be desirable, if by employing appropriate methods and materials, the notch sensitivity to impact loading of high heat polycarbonate could be reduced and its impact strength and toughness could be transformed into a characteristics as valuable as, and existing simultaneously with, its resistance to thermal deformation, particularly when the high heat polycarbonate is blended with a polyester. A feature of this invention, therefore, is a blended composition of a high heat polycarbonate and a polyester which has a desirably high level of impact resistance and resistance to thermal deformation in combination with a desirably high level of chemical resistance and which is easily processed.